Known optical sensors comprise e.g. a transmission unit for the transmission of detection light into a detection region, a receiver for receiving light reflected or remitted from the detection region and an evaluation unit for evaluating the reflected or remitted light. In this respect it can be e.g. a scanner system such as are described in DE 198 00 968 C2, EP 1 980 871 B1 or DE 39 08 273 C1 or distance measuring sensors which can, e.g. determine the distance from the light duration which a light signal transmitted into a monitoring zone takes after reflection from an object in the monitoring zone back to the sensor.
Other optical sensors are designed as light barriers or light grids which detect the presence of an object in a monitoring zone by interruption of a light beam or reflection of the light beam from the monitoring zone.
For monitoring the functionality, in particular for the reliable object detection and object localization in a safety system a control of the most important power functions is required. In known solutions test objects are provided in the monitoring zone of the sensor for this purpose and it is checked before or during the measurement whether these test objects are correctly observed.
The presence of such test objects in the monitoring zone can, however, even be detrimental to the monitoring itself.